Photographs and video from Christchurch, a graceful 19th-century city of nearly 400,000 residents, showed people running through the streets, landslides pouring rocks and debris into suburban streets and extensive damage to buildings. Witnesses told of watching the spire of the iconic Christchurch Cathedral come crashing down during an aftershock. One witness called it “the most frightening thing of my entire life,” and television video showed a person clinging to a window in the cathedral’s steeple.

Officials warned that the death toll was likely to rise as scores of people were still missing and feared trapped in the wreckage of several buildings that were flattened by the 6.3 magnitude earthquake or the aftershocks still rocking the city.

“I think we need to prepare ourselves in this city for a death toll that could be significant,” Mayor Bob Parker told reporters shortly after declaring a state of emergency and ordering the evacuation of the city center. “It’s not going to be good news, and we need to steel ourselves to understand that.”

Hundreds of frightened residents crammed into temporary shelters. Mr. Parker warned residents to prepare for a night without electricity and running water. Food and drinking water were being brought into the city overnight, he said.

The rescue mission was further complicated by repeated strong aftershocks and wet, chilly conditions overnight.

Prime Minister John Key said the extent of the devastation was unknown, but that New Zealand had witnessed “its darkest day,” and one of its worst natural disasters.

“It’s an absolute tragedy for this city, for New Zealand, for the people that we care so much about,” he told TVNZ, the national television broadcaster. “People are just sitting on the side of the road, their heads in their hands. This is a community that is absolutely in agony.”

A number of makeshift triage centers and emergency clinics were set up across the city to handle the influx of injured people. Officials said the city’s largest medical facility, Christchurch Hospital, was bracing for multiple casualties. Some victims have been airlifted to hospitals outside the earthquake zone.

By Tuesday afternoon, officials said there were no ambulances available in the city, all were tied up with urgent calls. Video from the scene showed office workers loading their injured colleagues into station wagons and four-wheel drives because of the lack of emergency vehicles.

The Christchurch Airport was closed and said on its Web site that it would reopen Wednesday morning only for domestic flights.

Video from the scene by 3 News New Zealand showed emergency crews pulling shaken and injured victims from damaged buildings, including one four-story structure, the Pine Gould Guinness building, which was nearly flattened. The top three floors of the building, a 1960s-era structure, had collapsed as terrified workers huddled under desks. Video showed one woman clinging to the roof as emergency workers raised a crane to rescue her.

“There was a guy on the second floor who was buried up to his waist in concrete and stuff,” one man, who escaped the Pine Gould building, told 3 News. “I’ve never seen anything like it.”

In another building, residents rappelled to safety from a broken window after the stairwell in their 17-story building collapsed.

Officials gave no clear estimates of how many people might have been trapped in rubble, and there were conflicting reports about the number missing in the chaos late Tuesday.

Some witnesses reported seeing people inside the Christchurch Cathedral when its spire collapsed, but it was not clear if anyone was killed. The Associated Press and other news outlets reported that up to 23 Japanese exchange students were trapped in their language school, which was located inside the devastated CTV building in downtown Christchurch.

The earthquake hit the country’s South Island just before 1 p.m. local time, and the United States Geological Survey said it was part of an aftershock sequence from a magnitude 7.1 earthquake that rocked the same area in September, but caused no casualties.

“There is more substantial damage to buildings than there was during the original earthquake,” the civil defense minister, John Carter, told reporters in the capital, Wellington. Tuesday’s tremor was centered about six miles from downtown Christchurch, and was only about three miles underground, possibly making it more destructive.

Though it was shorter in duration and lower in magnitude, many residents said the earthquake felt more violent than September’s.

Several news outlets reported extensive devastation to the nearby seaside town of Lyttelton, nearest the epicenter of the quake.

According to The Associated Press, the earthquake dislodged 30 million tons of ice from the Tasman Glacier in the Southern Alps that slide into a lake, creating waves up to 11-feet high.

The A.P. also reported that an American delegation of 43 government, business and community leaders in the city for a United States New Zealand Partnership Forum meeting were thought to be safe. It said that nine congressmen who attended the meeting had left the city before the disaster.

“The earthquake itself was quite violent, a lot of movement,” said Jason Tweedie, a 40-year-old Christchurch resident who was sitting in his four-wheel drive vehicle when the earthquake struck. “It felt like there were about 10 people shaking the side of it, all at once, it was so much force.”

The force of the earthquake pushed thousands of gallons of water and silt into the streets, Mr. Tweedie said, and in some places the road appeared to open up and swallow several cars in his beachside neighborhood of New Brighton.

Julian Sanderson was in his apartment on the first floor of an old brick movie theater when the walls and ceiling began to crumble around him.

“When it all stopped, I had to kick out the front door to get out,” Mr. Sanderson, 41, said by telephone, standing in front of his nearly collapsed building. “I used to work in that building making furniture, but everything has just changed. What we have now is the clothes that we’re wearing.”

Kevin Drew contributed reporting from Hong Kong.

This article "Scores Killed in New Zealand Earthquake" originally appeared at The New York Times.